The invention relates to articles, the shape of which and the surface of which are designed in a specific manner to achieve an aesthetic effect.
The invention relates especially to a decorated article which exhibits a transparent body which has a specific shape and on the surface of which a layer system is provided to generate interference effects.
Technical applications for optical components are known and conventional. Examples are high-efficiency mirrors, filters and beam splitters. A survey is included in H.A.M. Macloed, Thin Film Optical Devices, in "Active and passive thin film devices", Academic Press 1978 (hereafter, reference D1)
A number of components may be produced only with the aid of interference layers. It is possible to cultivate virtually all physically non-forbidden optical properties with the aid of interference layer systems. The possibilities extend from complete dereflection to a mirror which reflects more strongly than a silver surface; from a narrow-band transmission filter to a band pass with steep edges. The dependence of the optical properties transmission T and reflection R upon the wavelength results in the fact that the layer system appears to be coloured. In particular, it is easy to achieve powerful, coloured reflection using interference layer systems; this is not possible or is possible only with difficulty when using other means.
Conventional dyeing takes place by addition of substances which absorb a specific wavelength range. The article then appears in the colour of the non-absorbed wavelengths. This mechanism takes place mainly upon the transmission of the light, scarcely at all in reflection. Absorbing coloration is characterised in that a part of the light is destroyed. An article coloured in this manner has a dark effect. The greater the purity and depth of the colour, the more light must be absorbed and the darker is the effect of the article. This effect makes itself disadvantageously noticeable especially in circumstances in which the colour is to be employed as a decorative element to achieve an aesthetic effect. The absorption is a property of the substances employed, so that the available colours are limited by the number of appropriate substances. Since what is involved is absorbing coloration, as a rule the mixing of various substances gives an impure mixed colour.
The application of absorption-free interference layer systems brings the following advantages:
production of any desired clear colours is possible, PA1 powerful, coloured reflection, PA1 bright colours, no loss of light. PA1 European Patent Application, Appl. No.: 85304031.9, Publ. No.: 0 165 021 (hereafter reference D2) and PA1 DE-OS 3635567 (hereafter reference D3). PA1 (a) the layer system comprises a sequence of at least three interference layers of varying refractive index, PA1 (b) the surface of the transparent body is designed so that surface regions are present, the surface normals to which differ in all three spatial components, PA1 (c) the layer system is disposed directly on the surface regions and PA1 (d) the transparent body is designed in such a manner that in the customary position of use of the body at least one coated surface region can be viewed through a second coated surface region.
Nevertheless, the absorption-free interference layer systems have not to date been used for decoration. It is to be assumed that the decisive factor concerning the non-use is the following: The colour effect of absorption-free interference layer systems shows a marked dependence upon the illumination conditions. In particular, it is harmful for an application that the colour effects almost disappear under usual, partly uniform illumination.
For explanatory purposes, the term "depth of colour" K will be used. Light of intensity I impinges upon the eye of the observer. The change in the intensity with the wavelength is of decisive importance to the colour perception. Maximum intensity Imax, and minimum intensity, Imin, occur within the visible range of the spectrum. The function K=(Imax-Imin)/Imin can be taken as a measure of the depth of colour, provided that the extreme values are not so close that the eye integrates with respect to the wavelength. In the event that the intensity does not fluctuate, Imax-Imin=0, the value 0 emerges for K. In fact, in this case the light appears white (colourless). The eye "measures" relatively, so that the ratio of intensities is computed in K. A large value of Imin reduces the value of K, so that it is taken into consideration that a basic intensity existing at all wavelengths "whitens" the colour.
of the coloration of an article, the decoration of which consists in the coloration, it is to be required that the colours become effective under many illumination conditions.
1st case: pure reflection
In the case of pure reflection, layer systems and even single layers (lustres, soap bubbles) show great depth of colour. This is caused by a low value of Imin. It is not necessary to make any effort to achieve adequate depth of colour in reflection. However, pure reflection occurs very infrequently.
2nd case: pure transmission
In pure transmission, acceptable depth of colour can be achieved only with threefold layers (Table 1, below). It does not present any difficulty to achieve any selectably great depth of colour by design of the layer system. Pure transmission occurs more frequently than pure reflection. In most cases, it is sufficient to view a light source through the article. The brightness of a light source is, in comparison with the surroundings, frequently so great that to an approximation it is possible to refer to pure transmission.
3rd case: reflection and transmission at the same time.
In this case, the depth of colour is a function of the ratio of the causative intensities. FIG. 1 shows a typical illumination. A transparent article G, which exhibits an interference layer S, is situated above a base surface U (e.g.: the surface of a table) and is viewed obliquely from above (indicated by the eye symbol. FIG. 3 shows the quantities which are employed for the computation of the intensity impinging on the eye. The intensity is made up of intensity A incident from the left, multiplied by the transmission T, and the intensity B incident from the right, multiplied by the reflection R. ##EQU1## Table 2 (below) shows the depth of colour K as a function of V and R for the case concerning FIG. 1. The uncoated rear surface of the article is disregarded. The maximum reflection 60%, 81%, 93%, 96% corresponds to 3, 5, 7 and 9-fold layers (Table 1). It emerges from Table 2 that in the case of uniform illumination of the base surface (A=B or V=1) the depth of colour is precisely zero. This entire disappearance of the colour also occurs under real conditions; that is, irrespective of other light sources and illumination conditions in the room, as long as only the base surface is uniformly illuminated. Table 2 also shows the values of K for non-uniform illumination. The average value of K at differing values of V is a measure of the decorative effect under customary illumination conditions. It is of interest that the average value of K cannot be substantially increased by using a large number of layers.
The fact that the colour is weak under customary illumination conditions and can in some cases entirely disappear is a considerable defect which prevents the use of interference layers for the imparting of colour. Awareness of this defect forms part of the state of knowledge. Various proposals were made for the purpose of alleviating the defect:
A symbol-generating optical interference device for authenticity verification is propose in reference D2. The danger that the interference colours are not visible is to be prevented in that entire layer systems are applied one on top of the other. In fact, the average depth of colour increases with an increasing number of layers (increasing maximum reflection) to some extent, see Table 2. However, a great expenditure is made for this purpose. The production expenditure increases excessively with the number of layers, because layer defects are additive. The gain in depth of colour is very small. Even with an arbitrarily large number of layers, the colour cannot be prevented from entirely disappearing under uniform illumination (V=1).
Reference D3 is based on the observation that even low depths of colour are sufficient for decoration if it is ensured that different colours are viewed at the same time. What takes place is an intensification of the contrast, and thus of the decorative effect, where two different colours are compared with one another; even where the respective depth of colour is very low. However, the specification according to reference D3 presumes a non-disappearing depth of colour. In the event of uniform illumination, all colours disappear, so that in the circumstances it is no longer possible to make any comparison of different colours. Furthermore, the excessively low depth of colour in the vicinity of V=1 can scarcely be alleviated by the specification according to reference D3.